1. Field of the Invention
This invention relates to improvements in a cationic electrodeposition paint composition, and more particularly to a cationic electrodeposition paint composition which is high in an ability for covering an edge portion of a metal plate and high in weatherability.
2. Description of the Prior Art
Hitherto a cationic electrodeposition paint has been prepared by neutralizing a variety of basic resins such as basic epoxy resin, acrylic resin and polyurethane resin with organic acids, and followed by diluting it with water. In order to accomplish an electrodeposition painting, a metal object (such as steel plate) to be coated with a paint is dipped in the cationic electrodeposition paint and serves as cathode. Upon passing direct current through the metal object, a paint film which is insoluble in water is deposited on the surface of the metal object and thereafter baked to be hardened. Such an electrodeposition painting have been extensively used in industry to obtain a coated paint film which is high in weatherability and appearance while providing a corrosion resistance to the metal object. Additionally, a variety of compositions for such an electrodeposition paint have been proposed.
The cationic electrodeposition paint usually includes as a main component a cationic resin derived from an epoxy resin (referred hereinafter to as "epoxy cationic resin") or a cationic resin derived from an acrylic resin (referred hereinafter to as "acryl cationic resin". The electrodeposition paint including the former epoxy cationic resin is excellent in corrosion resistance to the metal object (steel plate) coated with the paint but inferior in weatherability. On the contrary, the electrodeposition paint including the latter acrylic cationic resin is excellent in weatherability but inferior in the corrosition resistance to the metal object (steel plate) to be coated with the paint. Such a fact has been known in the art.
In order to achieve both a high weatherability and a high corrosion resistance to the metal object, a cationic electrodeposition paint composition including both the epoxy cationic resin and the acryl cationic resin has been proposed as disclosed, for example, in Japanese Patent Publication No. 61-44906, and a Japanese technical magazine "The Industrical Coating" (by Toryo Hochi Shinbunsha) No. 87, Pages 38, published on Jul. 15, 1987 and No. 95, Page 42, Nov. 15, 1988. However, these publications are silent in improving an ability for covering the edge portion of the metal object (steel plate).
In this regard, it is effective for improving the edge portion covering ability, to control the rhelogy of a molten coated paint film in a baking step, in which the melt viscosity is raised to lower the surface tension. As a measure for controlling the rheology, using a pigment having a large oil absorping ability has been proposed as disclosed for example in Japanese Patent Provisional Publication No. 63-62897. As another measure for the same purpose, using cross-linked resin particulate (so-called micro-gel) has been proposed as disclosed for example in Japanese Patent Provisional Publication No. 63-63761 and the Japanese technical magazine "The Industrial Coating" No. 95, pages 33 to 35. However, with even such measures, the weatherability and the edge portion covering ability (the corrosion resistance to the edge portion of the metal object) cannot be satisfied.